of/fdt: introduce of_get_flat_dt_size

Add a wrapper function to retrieve the FDT size from the FDT header. This
is primarily to avoid libfdt include paths for the whole kernel.

Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Stephen Chivers <schivers@csc.com>